I am now finalising the circuit and starting to draw a PCB. It will be adjustable to the extent of replaceable resistors in IC sockets, not by trimmers. The subsonic filter will be simplified with only 6 dB/octave

But is a phase control necessary or would a simple invert switch be enough? Perhaps the phase control also does more harm than good? I have been looking at this: Project 103 - Subwoofer Phase Controller .

Another approach would be to make a true crossover network incorporating the high-pass characteristics of the "satellite" speakers. See Siegfried Linkwitz for details. Phase corrections and such are bandaids, and a defined crossover is just as easy to do for DIY.